Transport of mercury species in aquatic media is strongly affected by sorption processes on both organic and inorganic particles, and such mass transfer reactions are governed by speciation of this contaminant in the environment. The present research investigates the uptake of inorganic HgII species by sorbent materials based on polysiloxane-immobilized amine or thiol ligands, with the goal to highlight the effect of speciation on the sorption processes. Mercury binding was studied as a function of pH and chloride concentration to cover a wide range of species, including some among the most widely encountered forms in natural medium (HgCl42−, HgCl3−, HgCl2, Hg(OH)2). It was found on the one hand that all these species are liable to be removed from solution by strong binding to silica gels grafted with mercaptopropyl groups. On the other hand, the use of silica bearing aminopropyl moieties has led to the selective accumulation of anionic chloro-complexes of HgII in acidic medium in the presence of high chloride concentration and to the uptake of HgII hydroxide around neutral pH, while HgCl2 cannot be adsorbed on this material. The former reaction involves electrostatic interactions between HgCl42− or HgCl3− and the ammonium form of polysiloxane-immobilized amine ligands, and the latter is driven by complexation of HgII species to the unprotonated amine groups. The interest of electrochemistry at carbon paste electrodes modified with these adsorbents for characterizing the sorption processes was also described by applying the following sequence “voltammetric detection subsequent to open-circuit accumulation”. This approach allows the in situ determination of the amount of mercury in the organic–inorganic hybrid phase, as a function of speciation in solution and without significant modification in the solution-phase concentrations, so that the adsorption and ion-exchange isotherms can be obtained very easily and without requiring additional treatment of the solid material.